If we're going to trust these remailers then we need to do things properly. Key goes into the crypto processor, never comes out. Means someone can't just seize your server and image it then use that image to decrypt all traffic that passed through. If they want to try and get it out, fine but they'll need a guy with an Electron microscope to do so and they'll likely trip the tamper measures and bye bye key. If you're particularly paranoid you can even destroy your copy of the key once you've loaded it, this might mean changing your key if you have to move servers but it means that the service you offer is truly tamper evident. Plus you also have the added bonus that a dedicated hardware security module is usually quicker than your processor at doing encryption/decryption.

Do they really store ALL email traffic, or just profile and store from selected accounts?
The 3GB of mails from my GMail consisting of newsletters and college projects, and millions of other accounts like mine: arent they essentially useless and a waste of space for them?

Suppose you had a yottabyte of disk storage. 3GB isn't just a drop in the bucket, it's not even a grain of sand at the beach.

Car Analogy: Most of us break the odd traffic law every now and then. Very rarely, does anybody get caught. At the instant Officer Friendly pegs you on radar doing 35 in a 30 zone, he'd very much like to be able to check your driving history. If there were a giant database of everyone's GPS logs, he could tell whether you were just in a hurry that morning, the sort of driver who usually drives precisely 4 (or 9) miles an hour over the posted speed limit, or if you do 120 in a 60 zone whenever there aren't any cops around. If Officer Friendly had access to that data, he'd be better able to judge whether or not to pull you over.

For speeding, it's not worth logging the movements of every car and correlating them with local speed limits at the time the log was written.

For other things, it probably is.

From NSA's point of view, right now your gmail account is noise. But everyone's political views change over time as a natural part of the process of growing up. Sometimes things go wrong, and perfectly normal people who hold perfectly normal views turn into monsters. There's a 99.99999% probability that you're not one of them. But for the sake of 3 lousy gigs out of a yottabyte, there's a 100% chance that someone's 3GB of noise will contain signal.

Since they don't posess a time machine that can peer into the future, they don't, and can't, know whose 3GB-of-noise will eventually contain a signal 20 years from now. But 20 years from now, they will have a time machine that can peer back 20 years into the past.